U.S. Pat. No. 2,588,770 describes such a method of stunning animals before slaughter. The animals are decompressed either until they are stunned, or until they die. For stunning the pressure in the decompression chamber is dropped in a single step to a predetermined subatmospheric pressure which is maintained until the animal loses consciousness. For killing the pressure is subsequently dropped again in a second step until the animals die from oxygen starvation.
The disadvantage of killing the animals before slaughter is that the animal's circulatory system will not assist at least the initial bleeding of the body. Draining the blood from an animal that is already dead is time consuming so that frequently in the interest of saving time such bleeding is not completed.
Another disadvantage of this method is the animals are subjected to substantial stress during the initial decompression stunning phase. The natural instincts of the animal call up considerable fear with the associated physiological changes, including changes in blood chemistry and cramping, the former can affect the taste of the meat and the latter can further complicate bleeding of the cadaver. Even though the known method is intended to avoid these effects in the animals to be slaughtered by oxygen deprivation, it does not take into effect the physical and even psychological effects created by an uncontrolled decompression. In fact the effect can be as great as the stunning methods employing electric shock or carbon dixide in as far as producing pain, stress, and cramps in the animals.
In addition tests have shown that a one-step drop in the air pressure to stunn the animals is disadvantageous. In itself a sudden pressure drop to a low subatmospheric pressure does serve to effectively stun the animals because the oxygen deprivation is extreme so that all reflexes are lost. Such rapid decompression, however, can actually damage the tissues of the animals. In particular lung hemorrhaging is a possibility as well as damage to other internal tissues. Thus it is normally necessary to slow down the process to avoid such tissue damage, and of course such slowing increases the other stresses the animal is subjected to.